Hydrophobic silica particles possess physical properties that are useful in a number of applications requiring a high degree of dispersibility, including use in toner compositions, as antiblocking agents, as adhesion modifiers, and as polymer fillers. Untreated silica particles are hydrophilic due to the presence of silanol groups on the surface of the untreated silica particles. By treating hydrophilic silica particles, the hydrophilic nature of the silica particles can be reduced, thereby imparting varying degrees of hydrophobicity to the particles.
Many different methods are known for treating the surface of silica particles. Frequently, aqueous dispersions of silica particles are treated with reagents to introduce hydrophobic functional groups onto the silica surface. In order to prevent condensation of the silica particles into gels or large agglomerates, the aqueous dispersions of silica particles are typically stabilized by control of the pH. Prior art processes typically utilize basic aqueous dispersions of silica particles, wherein stability is maintained at a basic pH. However, the pH of the aqueous silica dispersions affects the surface chemistry of the silica particles in reaction with treating agents and limits the type of modifications possible at the particle surface. European Patent Application Publication EP 1657283A1 describes the surface treatment of acidic aqueous dispersions of silica particles with a silazane and optionally with a combination of a silazane and a siloxane. However, such a process limits the surface treatment of the silica particles to introduction of a single type of functional group, namely, a trimethylsilyl group.